The present invention relates to a transfer impedance testing fixture for detecting the shielding characteristics of an electromagnetic wave shielding material in terms of the transfer impedance used for preventing electromagnetic wave interference (EMI).
As electronic devices have been recently become popular, the electromagnetic wave noise emitted from the electronic devices causes problems. For instance, the electromagnetic wave noise emitted from electronic devices causes other electronic devices to malfunction and adversely affects the human body. To avoid these problems, various types of electromagnetic wave shielding materials have been developed. For development, it is necessary to determine the shielding characteristics of the electromagnetic wave shielding materials. Conventionally, to evaluate the shielding characteristics of these materials, the following testing methods have been proposed.
(1) While the emission power of the electromagnetic wave is set constant, induced electromotive force P.sub.1 is measured by a receiving antenna when there is positioned a sample of an electromagnetic wave shielding material between a transmitting antenna and the receiving antenna. Similarly, induced electromotive force P.sub.0 is measured by the receiving antenna when there is no positioned sample of shielding material. With the following calculation, the shielding characteristics of the sample are evaluated. EQU Se'=10.multidot. log P.sub.0 /P.sub.1 ( 1) PA0 (2) At the center of a coaxial testing fixture, an output power P.sub.1 is measured when a sample is inserted between an outer conductor and an inner conductor, and another output power P.sub.0 is measured when no sample is inserted. The above Se' is calculated. PA0 (3) A TEM cell for emitting the electromagnetic wave and a TEM cell for detecting it are combined via a window. A sample is positioned at the window to measure a detection power P.sub.1.
Another detection power P.sub.0 is measured with no sample. The above Se' is calculated.
However, all of the above three methods require correction factors since the value of Se' depends on the size of the sample. The above methods are therefore inappropriate to accurately evaluate the shielding characteristics of the electromagnetic wave shielding materials.
On the contrary, a new method has been proposed for evaluating the shielding characteristics by testing the transfer impedance (See, for example, SAE ARP-1705 "Coaxial Test Procedure to Measure the RF Shielding Characteristics of EMI Gasket Materials").
FIG. 12 shows a system for measuring transfer impedance in which a current is directed to flow through the sample wall of the electromagnetic wave shielding material T with current set as I and voltage set as V, respectively. The transfer impedance Zt is calculated. EQU Zt=V/I
The shielding characteristics are then calculated as follows. EQU Se=K.multidot.Zw/Zt (2)
K is a coefficient depending on the measurement system impedance of the testing fixture and Zw is a wave impedance (For example, K=50/120.pi. in a 50.OMEGA. system).
According to this method, as compared with the aforementioned three methods, measurement variations due to the size and the shape of the sample of electromagnetic wave shielding material does not occur, and the system operates without any necessity of correction. The measured value Se is suitable for evaluating the shielding characteristics of the shielding material.
The conventional system for measuring the transfer impedance as disclosed in the above-referenced article is directed mainly to measuring a gasket, so that it is not suitable for measuring the transfer impedance of any electromagnetic wave shielding material of sheet-like or one foil-like form.
Basically, the conventional system is too complicated to easily set a sample to measure. Further, impedance matching between an outer conductor and an inner conductor cannot be established in the conventional system, resulting in the disadvantageous effect of reflection by a high frequency.